The setting is semi modern urban fantasy where there are mercenaries with the task of slaying monsters on the unpopulated areas, steel is the first metal to go when it comes to making weapons on large scale or for those who don't have the money for better materials. But because almost everyone and most of monsters can potentially bend fire, spit corrosive fluids etc. Making so the best weapon material that one those mercenaries could use must be better than classic spring steel in those cases; My best bet is to introduce cobalt weapons, I've read about the properties of cobalt and somewhat they match, but I'm not that sure of how good it would be for weapons like swords. Did a bit of research and found Elgiloy which is an alloy that's used for springs and from what I know spring metal works well especially for swords. But I'm not sure of how good would that work.

  • $\begingroup$ What does cobalt do which marine stainless steel doesn't? $\endgroup$
    – AlexP
    Feb 27 '20 at 15:45
  • $\begingroup$ Wolfram or Titanium? $\endgroup$ Feb 27 '20 at 15:46
  • $\begingroup$ @AlexP Cobalt can mechanically work at high temperatures $\endgroup$ Feb 27 '20 at 15:51
  • $\begingroup$ @SZCZERZOKŁY not sure about wolfram but titanium isn't good for edges $\endgroup$ Feb 27 '20 at 15:52
  • $\begingroup$ @Dawnstarwatcher I've been always a fan of stabby-longy-distanty-thingy like arrows or spear rather than swords. $\endgroup$ Feb 27 '20 at 15:57

The main advantage of using iron alloys for making swords is that the properties of the material can be locally tuned to improve the global performance of the blade.

When you look at how a blade works, you want to have:

  • a hard surface, to be able to resist the impact with the target
  • a tenacious core, to be able to withstand the solicitations imparted by the blow

Now, a single alloy can be hard and brittle or tenacious and softer, and you see that you will have to compromise too much to make a good sword.

Using iron based alloys instead one can use I.e. quenching or carbon hardening to have a hard surface while maintaining a tenacious core, dramatically improving the performances of the blade.

From what I see, cobalt and its alloys are generally hard and resistant to high temperature, but that alone would make the cobalt blade brittle. And you don't want a sword that shatters on impact, don't you?

  • $\begingroup$ why would high temperature resistance make it brittle? $\endgroup$ Feb 27 '20 at 16:25
  • 3
    $\begingroup$ @Dawnstarwatcher, hardness is usually coupled to brittleness. Temperature resistance has nothing to do with it. $\endgroup$
    – L.Dutch
    Feb 27 '20 at 16:32
  • $\begingroup$ sorry misread that part, I understand that the material being brittle is a problem, but is hardness proportional to how brittle it is? or there's a property related to it? $\endgroup$ Feb 27 '20 at 16:47
  • 1
    $\begingroup$ @Dawnstarwatcher yes, basically hardness = brittleness in that the properties that make something 'hard' (in this case, able to hold a sharp edge) make it unable to flex under impact, which prevents distribution of force across a greater volume/area of the blade. Steel is great because it can be hard in one place and softer elsewhere in a single piece which lets you mix and match your physical properties in a way that most other materials don't permit. $\endgroup$ Feb 27 '20 at 19:48

In additional to its brittleness, the oxidation products of cobalt are toxic. Not in a "poisoned sword that keeps on poisoning" sort of way, more in a "use this thing long enough, and you'll get deathly ill and may die" manner.

Now, there are cobalt alloys (with iron, which makes them steels) that work well, but in general, you want a carbon steel for weapons. It can be hardened by heat treatment, and tempered to make the hard edge and tough core that makes a sword durable. Adding a little molybdenum helps a lot for toughness, while cobalt can help with corrosion (not as much as chromium and nickel, though -- those go into stainless).

And contrary to the movies, there are no acids that will eat away your blade in the blink of an eye. Get a splash of concentrate nitric, sulfuric, or hydrochloric acid on the blade, and if you don't just let it sit there and eat away at the surface you'll get local darkening and etching (the kind that brings out the grain structure in a polished surface) -- not wholesale destruction.

  • $\begingroup$ I should had put it more clear on the corrosion part, never intended it to be like in the movies, but more like how long it would take for it needing some maintenance. The parts of the toxic byproducts makes sense but wouldn't it help having it protected from it? coating it with oil, people do it with steel why wouldn't it happen with cobalt. $\endgroup$ Feb 27 '20 at 17:03

It would definitely be hard to do.

just finding the material is going to be way harder than finding iron.

I found this through Wikipedia, definitely worth checking out this graph Elemental Abundance

You can see that Iron (Fe) has about 10^5 atoms for every 10^6 atoms of Si, whereas Cobalt (Co) has about 10^1 atoms per 10^6 atoms of Si.

1,000 Atoms of Fe for every atom of Co.

Let's talk melting point now.

Iron has a melting point of 1,538C. Cobalt has a melting point of 1,495C. that is only about a 40-degree difference. Therefore we will say it has no impact on anything essential.

Cobalt has a density of 8.86g/cm^3 whereas Iron has a density of 7.874g/cm^3. Let's remember that casting cobalt means you're probably going to have to anneal it as well.

after all of this work, you'd think it would be worth it right?

Absolutely not.

Cobalt is extremely brittle so if you decide to try to sharpen it we can assume it would just shatter into a million pieces.

Shortened Answer:

You'd be putting in too much time and effort to A) Mine cobalt, B) Melt it, C) anneal it, and D) Sharpen it, just to be presented with a heavy blade that would slowly poison you over time. Not worth all that effort if it's gonna kill you.

  • $\begingroup$ Please link to where you got the image from. Is it showing Earth abundances? $\endgroup$
    – Spencer
    Feb 28 '20 at 0:09

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